Meiosis

Genes Scientists in the 1900s knew that genes carried the characteristics of different organisms. Where were these genes? What cellular process was responsible for the different characteristics of genes?

Chromosomes Strands of DNA and protein inside the nucleus in eukaryotic (animals and plants) cells Carriers of genes Genes are located in specific positions on chromosomes

Diploid Cells Diploid – “two sets” Haploid Diploid – “two sets” The diploid cells of most adult organisms contain two complete sets of inherited chromosomes Diploid

Homologous Chromosomes in Diploid Cells These two chromosomes are known as homologous or homologues

Diploid Number Diploid number in fruit flies is 8 2N = 8 N = the number of chromosomes in the parent sex cell (sperm or egg) Therefore, how many chromosomes does a fruit fly’s sperm cell contain?

Haploid Cells Haploid Some cells contain only a single set of chromosomes, and therefore a single set of genes These cells are called “haploid” cells Haploid – one set Diploid

Haploid Cells The gametes (sex cells) of sexually reproducing organisms are haploid For fruit flies, the haploid number is 4 N = 4

Sex Cells are Haploid If most cells in the body undergo mitosis and are diploid, then the sex cells that are haploid need to undergo a different process. This process is called meiosis

Mitosis Review Before we start learning the steps of meiosis, let’s review the steps of mitosis

Mitosis Review Prophase – genetic material inside the nucleus condenses and the duplicated chromosomes become visible. Outside the nucleus spindles start to form

Mitosis Review Metaphase – centromeres of the duplicated chromosomes line up across the center of the cell Spindle fibers connect the centromere of each chromosome to the pole of the spindle

Mitosis Review Anaphase – chromosomes separate and move along spindle fibers to opposite ends of the cell Telophase – chromosomes, which were distinct and condensed, begin to spread out into a tangle of chromatin

Phases of Meiosis There are two stages of meiosis, each with four phases Meiosis I Prophase I, Metaphase I, Anaphase I and Telophase I Meiosis II Prophase II, Metaphase II, Anaphase II, and Telophase II

The Stages of Meiosis Meiosis I Meiosis II

Meiosis I Just prior to meiosis I, the cell undergoes a round of chromosome replication during interphase I. Each replicated chromosome consists of two identical chromatids joined at the center.

Prophase I The cells begin to divide, forming a structure called a tetrad, which contains four chromatids

Prophase I As homologous chromosomes (corresponding chromosomes from male and female) pair up and form tetrads, they undergo a process called crossing-over

Crossing-over in Prophase I First, the chromatids of the homologous pairs cross over one another Then, the crossed sections of the chromatids are exchanged

Crossing-over and Alleles Crossing-over is important because it produces new combinations of alleles in the cell Alleles – one of a number of different forms of a gene This is what leads to genetic diversity

Crossing-over

Metaphase I As prophase I ends, a spindle forms and attaches to each tetrad During metaphase I, paired homologous chromosomes line up across the center of the cell

Anaphase I During anaphase I, spindle fibers pull each homologous chromosome pair toward opposite ends of the cell When anaphase I is complete, the separated chromosomes cluster at opposite ends of the cells

Telophase I During telophase I, a nuclear membrane forms around each cluster of chromosomes Cytokinesis follows telophase I, forming two new cells

Telophase I & Cytokinesis

Meiosis I Meiosis I results in two cells, called daughter cells. These two cells have a haploid number of chromosomes. The chromosomes are still duplicated.

Meiosis II The two cells produced by meiosis I now enter a second meiotic division Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II The steps of meiosis II look just like the steps of mitosis

Prophase II As the cell enters prophase II, their chromosomes become visible

Metaphase II During metaphase of meiosis II, chromosomes line up in the center of each cell.

Anaphase II As the cell enters anaphase, the paired chromosomes separate.

Telophase II and Cytokinesis Each of the four daughter cells formed in meiosis II receives unduplicated chromosomes that are different from the versions in each other cell.

Meiosis II The four new daughter cells now contain the haploid number (N) of unduplicated chromosomes.

Gametes to Zygotes The haploid cells produced by meiosis II are gametes Gametes – sex cells Male animals - sperm Male plants – pollen grains Female animals – only one of the cells produced in meiosis is involved in reproduction Egg cells

Gametes to Zygotes Fertilization – the fusion of male and female gametes – generates new combinations of alleles in a zygote Zygote – fertilized egg The zygote undergoes cell division by mitosis and eventually forms a new organism

LETS MAKE A BABY!!!!

LETS MAKE A BABY!!!! X Y

LETS MAKE A BABY!!!! X Y

LETS MAKE A BABY!!!! X Y

LETS MAKE A BABY!!!! X Y

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

LETS MAKE A BABY!!!! Y X

Y X LETS MAKE A BABY!!!!

Y X LETS MAKE A BABY!!!!

Y X Awww! WE MADE A ZYGOTE!

Please stand by while the zygote goes through MITOSIS… X

Please stand by while the zygote goes through MITOSIS… X Y X

Please stand by while the zygote goes through MITOSIS… X Y X Y X Y X

Please stand by while the zygote goes through MITOSIS… X Y X Y X Y X Y X Y X Y X Y X

Please stand by while the zygote goes through MITOSIS… X

Y X Y X Get the point? Y X

Now we’ve got a GROWN-UP BOY… X

…WHO MEETS A GIRL!!! Y X X X

After dating for 12 years, they decide it is time to get married… X X X

Meanwhile, they’ve been going through MEIOSIS… X X X

…in their ovaries and testicles! Watch and learn… X Y X

X Y X

X Y X

X Y X

X B D d b a Y X A b D S Phase!

Y X X A b D B D d b a S Phase! Y X A b D X B D d b a

S Phase! Y X A b D X B D d b a Prophase I Y X X

Prophase I Y X X Metaphase I Y X X

Metaphase I Y X X Anaphase I Y X X X

Anaphase I Y X X Telophase I Y X X X

Telophase I X Y Prophase II X Y

Prophase II X Y Metaphase II X Y

Metaphase II X Y Anaphase II X Y

Anaphase II X Y Y Telophase II X X Y X X X X

Now we have sperm and eggs! Can you fill in the alleles? X X X Y X X X

It is time to make another baby… X A b D X B D a X d b a Y A b X d a b X A D b X D a B

Lets do some bookkeeping… Y X X DAD Phenotype: Genotype: Possible Sperm: MOM Phenotype: Genotype: Possible Eggs: Afro, Bl Eyes NoFro, Br Eyes AA, bb aa, Bb Ab aB, ab

And now for a Punnet’s Square… Y X X Possible Eggs aB ab Phenotypic Ratio: 50% Blue eyed fro 50% Brown eyed fro AaBb Aabb Ab Fro, Br Eyes Fro, Bl eyes Possible Sperm